Optical amplifiers are provided in optical links of an optical network in order to compensate for optical losses caused by various optical elements within the network. During normal operation of the network, the optical amplifiers are operated to control the per channel optical power so that it is maintained at a preselected target optical power; the optical amplifiers are operated in what is known as a ‘closed loop’, variable gain condition. When an optical channel is dropped or added, for fast restoration of a wide number of channels or channel upgrade, or when an upstream fibre is unexpectedly cut, a sudden change in the total optical power input to an optical amplifier can occur, known as a power ‘transient’. When a transient is detected it is known to switch operation of optical amplifiers to a “freeze” condition in which a constant gain is applied, known as an “open loop” constant gain condition, to manage the optical power transient. This keeps the power of the existing/surviving optical channels stable during the fast power variation at the input of the optical amplifiers caused by the power transient. For example, U.S. Pat. No. 6,757,099 describes an optical power transient control scheme for Erbium doped fibre amplifiers, EDFAs, in which the pump optical power supplied to the Erbium doped fibre is controlled based on a detected variation in the input power of the optical signal entering the EDFA. When the network is has reached a steady state operation, the amplifiers are unfrozen and returned to the ‘closed loop’ variable gain operating condition. When the amplifiers are restored to ‘closed loop’ operation there is a risk that an optical power oscillation will be created, with potential impact on traffic. This because the output optical power/gain that the amplifiers are set at in the freeze condition is often slightly different from the actual target figures for normal operation (due to the spectral position of the surviving channels, amplified spontaneous emission, ASE, noise in the amplifiers, and the accuracy of the gain value that the amplifiers are ‘frozen’ at). When the gain control loops of the optical amplifiers are closed, each optical amplifier sees the output power/gain error and tries to correct the error. This can cause optical power oscillations to occur, even when the error is small.